Fact or fiction: Random mating in field populations of western corn rootworm (Diabrotica virgifera virgifera LeConte) emerging on Bt and refuge corn plants
The western corn rootworm, or WCR, (Diabrotica virgifera virgifera LeConte) is the most significant pest of field corn (Zea mays) in the United States, and has recently expanded its range into Europe. Since 2004, hybrid corn containing Bt toxins targeting the corn rootworm complex have been heavily adopted and are now the primary control measure for this pest in North American corn production. The evolution of resistance is an ongoing concern, and to ensure Bt products will retain their usefulness, insect resistance management (IRM) tactics using various refuge structures have been adopted. One of the key tenets of the refuge strategy is that males and females emerging from Bt and refuge plantings mate randomly. A violation of this largely untested assumption would lead to acceleration of resistance development. To generate empirical field data on mating rates between beetles emerging from Bt and refuge plants, field cage studies using field populations of WCR in Indiana were utilized. Various refuge configurations were tested; all refuge plants were labeled using the stable isotope N15. This mark persists in the adult beetles after eclosion, allowing for collection and analysis of isotopic ratios of beetles in mating pairs. This approach was used to test the random mating assumption in Bt and refuge beetles collected from field cages. Other data collected include emergence rates, timing and sex ratios for each of the treatments. Results indicate that mating based on natal host may not be as important of a factor as initially thought. Mixed mating occurs at a high rate when there are higher numbers of susceptible rootworms even though the measured fitness parameters between Cry3Bb1 and refuge adults were significantly different (p< 0.05). The main indication from this study is that not enough susceptible individuals are produced from a 5% refuge-in-a-bag strategy which is the dominant form of refuge planting in the United States.
Krupke, Purdue University.
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